my car runs fine except for the crank sensor malfunctioning now. does not over heat til most of coolant is pushed out of reservoir. this can be noticed by heat going out and when you rev the engine temp goes down heat comes on. all fluids are clean. tail pipe exhaust is normal. i flushed the system. replaced the fluid. Did the head gasket test kit, passed (only steam). Pressure tested, held 19 psi all day. I let it idle and drove it for hours (dove it at 80+mph). the next morning i drove it a couple of miles. no over heating but i noticed the cup i put the overflow hose in was full of antifreeze.I parked it and left it. went back to it two weeks later and coolant system was still under pressure. i have never experienced this. and ive fixed a lot of cars. even northstars. have any one ever experienced this. please help i dont want to do a head job for nothing.

my car runs fine except for the crank sensor malfunctioning now. does not over heat til most of coolant is pushed out of reservoir and steam gets into system. this can be noticed by heat going out and when you rev the engine temp goes down heat comes on. all fluids are clean. tail pipe exhaust is normal. i flushed the system. replaced the fluid. Did the head gasket test kit, passed (only steam). Pressure tested, held 19 psi all day. I let it idle and drove it for hours (dove it at 80+mph). the next morning i drove it a couple of miles. no over heating but i noticed the cup i put the overflow hose in was full of antifreeze.I parked it and left it. went back to it two weeks later and coolant system was still under pressure. i have never experienced this. and ive fixed a lot of cars. even northstars. have any one ever experienced this. please help i dont want to do a head job for nothing.

Re: Please help deville pressure build up in coolant system.

went back to it two weeks later and coolant system was still under pressure.
that's NOT right -
the cooling system - when cold - is not pressurized -
pressure develops as the antifreeze gets heated and expands -
once the antifreeze cools back down - the pressure is gone -

the only possible way a cooling system can be pressurized cold -
is from exhaust gasses being pumped in due to bad head gaskets -

Did the head gasket test kit, passed (only steam).
try the test again -
there are several YouTube videos showing the proper way to use the equipment -
and the expected results -

the blue test liquid reacts with hydrocarbons - exhaust gasses -
ANY COLOR CHANGE - even a slight change to a lighter blue -
is a positive reaction -

Re: Deville pressure build up in coolant system.

i did the fluid stayed blue. i graduated auto tech a little while ago. i am pretty knowledgable mechanically. these northstars are tricky. the problem with pushing it hard then doing the block test is when you open it whatever is in there is released. also if exhaust gas is getting in it has to get back out the same way. the system held pressure with all spark plugs removed. i read somewhere earlier about a purge valve. ive done 3 NS head jobs before 2 successfully. neither did what this is doing. this one almost seems as though it has a air pocket or restriction. sorry for the extra posts. im new did think iwas putting them in the right place for a thread.

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im gonna try running hard and rigging a contraption to catch whatever vapor is in there and test it.
funny thing is when i fill it with fluid and after it is purged out. it stays full to the top. even now a 20 ounce cup purged out and it is full to the rim.

Re: Deville pressure build up in coolant system.

also if exhaust gas is getting in it has to get back out the same way.

Not necessarily. Remember, IF the system is being pressurized by exhaust gases, the cylinder pressure forcing those gases into the cooling system is FAR, FAR greater than the 19 psi produced by a cooling system pressure tester. A pressure tester will not have anywhere near enough pressure to force coolant back into the cylinder with a very small HG breach or in the case of a Northstar, a pulled thread on maybe one bolt which reduces clamping pressure on the head enough to leak a little when the engine is running.

There is no purge "valve" on the Northstar. There is a purge "line". It is open all the time. It is the 3/8" line that runs from the WP housing aka crossover manifold to the surge tank. Any air in the system automatically gets into the line and flows to the surge tank. If the tank gets over pressurized, as in the case of a HG breach, excess pressure will then be expelled overboard via the cap vent, as you well know.

Re: Deville pressure build up in coolant system.

im gonna try running hard and rigging a contraption to catch whatever vapor is in there and test it.
funny thing is when i fill it with fluid and after it is purged out. it stays full to the top. even now a 20 ounce cup purged out and it is full to the rim. 2000 deville

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thanks Ranger u absolutely right. i used my air compressor at plugs and pumped it well above 19 and listened for seepage. i chose 19 -21 to hold it at as i was taught and have diagnosed and traced down sources before. The NS is tricky. i had one where the itnterior surface of the head was not smooth. ran fine but as soon as u put pressure on it BAM! The car is mint with 9Ok owned by a ww2 sarg.

Re: Deville pressure build up in coolant system.

You're not filling the surge tank to the top, are you?

I can understand an imperfect head surface. That would make sense. You just have to keep in mind how the Northstar works (or more accurately, how it fails). Like I said, generally speaking it's not a HG "breach". It's usually the head loosing clamping pressure, and that won't be a lot with only 1 or 2 head bolt threads pulled. Enough to seep when you put a load on it, but nothing from a few psi by a tester. Even 125 psi shop pressure in a cylinder doing a leak down test probably would not be enough to lift the head enough to leak compared to the enormous pressure built up in a cylinder when the fuel mixture is ignited. And when I say "lift" it's so minute it's probably not measurable.

Re: Deville pressure build up in coolant system.

wow. i guess some xxxx comes from experience. i went and got the car. let more fluid out. checked the purge which was clogged. did the block test again and it passed. floored it. ran great. let it sit running then took it and floored it again. no pressure build up. the guy i bought it from thought maybe bad head but it actually had a bad water pump and he had it overfilled. wow. thanx guys.

Re: Deville pressure build up in coolant system.

yes i did. and it does. i bought it from a guy with possible head problem otherwise in pristine condition. but it drove way too long without overheating and when and you had to rev the motor to get heat. he had it full all the way to the top. i replaced the freespining water pump. and put it back like it was. wow never really thought about the fact that anitfreeze expands. i was about to rip it apart til i found yall page and seen the purge thing and decided to post. thanks guys (Ranger)i was literally minutes from ordering a 500 buck stud kit.